Re: [Qemu-devel] [RFC PATCH] spapr: support time base offset migration

[Andreas Färber]

Am 05.09.2013 15:39, schrieb Alexander Graf:
> 
> On 05.09.2013, at 15:36, Benjamin Herrenschmidt wrote:
> 
>> On Thu, 2013-09-05 at 14:37 +0200, Alexander Graf wrote:
>>
>>> Hrm, I think I'm starting to understand what this is about. So what we want 
>>> is
>>>
>>>  - timebase in guest
>>>  - timebase frequency in guest
>>>  - wall clock time in host
>>>
>>> That way the receiving end can then take the timebase and add (new_timebase 
>>> - old_timebase) * tb_freq to the guest's time base.
>>>
>>> Which gets me to the next question. Can we modify the tb frequency in 
>>> guests?
>>
>> No. It's architected at 512Mhz however since P7 I think. Not sure how we
>> did before, it's possible that P6 was the same (at least it's sourced
>> from more/less the same chip TOD facility).
> 
> I think we should transmit the tb frequency as well to at least allow us to 
> adjust if a later chip derives here.

Are you thinking of POWER8 having a different frequency than POWER8 in
compat mode? Because migration from one -cpu to another is not supported
elsewhere.

Even if we want to migrate from one POWER7 revision to another, we
should let the destination use the revision of the source (guest ABI!),
via property if need be. Anything else will lead to confusion as to what
is supported and what is not. That -cpu host is the default for
convenience shouldn't relieve admins/libvirt to think about sensible
setups like they have to on x86.

Andreas

> 
> But yes, without frequency adjustment I see where you're coming from. We 
> still only need the timebase the guest sees, not the offset. But we also need 
> the host wall clock to allow for adjustments :).
> 
> 
> Alex
> 


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